This invention relates to the utilization of geothermal energy, and more particularly to a process for deriving useful energy from geothermal brines.
In light of the problem of diminishing hydrocarbon fuel reserves, there is a developing emphasis to exploit alternative energy sources, such as geothermal energy. Geothermal energy is generated by producing steam, hot water, or hot aqueous brines from a subterranean geothermal reservoir and utilizing the produced fluid in conjunction with surface equipment, such as turbines and heat exchangers, to derive useful energy therefrom.
Most commercially exploited hot aqueous geothermal brines produced from subterranean geothermal reservoirs contain appreciable quantities of non-condensible gases, such as hydrogen sulfide and carbon dioxide. Not only will hydrogen sulfide create precipitation, scaling and corrosion problems within the treatment or power generation systems, but only very minute amounts of hydrogen sulfide can be acceptably discharged into the atmosphere. Also, as carbon dioxide is depleted from a subterranean geothermal reservoir, the partial pressure of carbon dioxide also decreases. Thus, when geothermal brines are produced from the reservoir, the reduced carbon dioxide partial pressure on the brine remaining in the reservoir causes the chemical equilibrium to shift so as to favor carbonate formation. As a result, carbonate scaling of the well and process piping and equipment occurs, usually as calcium carbonate scale.
Against this setting, several processes have been proposed for utilizing geothermal brine. One such process proposes to first withdraw and suitably treat non-condensible gases from geothermal brine and then flash the brine to steam. The remaining "waste" brine is reinjected into the subterranean geothermal reservoir. The steam is utilized to drive a turbine and then cooled and condensed in surface condensers. Substantially all of the hydrogen sulfide present appears with the non-condensibles in the exhaust vent from the after condensers and is converted to sulfur by any available process, such as the Stretford process. The steam may be scrubbed with condensate at a location upstream from the turbine to remove mineral impurities as well as some ammonia and hydrogen sulfide. The condensate can be disposed of by reinjection with the "waste" brine. However, it is noted that silica contained in the steam fed to a turbine deposits on the turbine blades necessitating turbine shutdown for cleaning, and that precipitated dissolved solids must be removed from waste waters before reinjection to prevent plugging of the reinjection wells.
Another proposed process utilizes the addition of oxygen to produced geothermal brines prior to entry into the first wellhead flash vessel to oxidize sulfide present in the geothermal brine to free sulfur or sulfate ion and further, to divert reinjection to a waste pond so as to remove silica solids prior to reinjection. It has also been proposed to treat vent gas from surface condensers to remove hydrogen sulfide therefrom by passing the gas through a reactor containing a packed bed of vanadium catalyst thereby converting hydrogen sulfide to sulfur dioxide which may then be discharged to the atmosphere. Still another prior art process discloses removing non-condensible gases, including carbon dioxide, from produced geothermal water, compressing the non-condensible gases and utilizing these gases as a gas lift to aid in production of the geothermal water. The carbon dioxide helps maintain chemical equilibrium in the water and thereby prevents, inter alia, calcium carbonate formation. However, none of these prior art processes offer a comprehensive process for deriving useful energy from the produced geothermal brine and concommitantly treating the brine to effectively eliminate scaling and corrosion associated therewith. Thus, a need exists for such a process.
Accordingly, it is an object of the present invention to provide a process for efficiently deriving useful energy from produced geothermal brine which will at the same time effectively and substantially eliminate attendant scaling and corrosion.
Another object of the present invention is to provide a process for deriving useful energy from geothermal brines in a substantially pollution-free manner.
A further object of the present invention is to provide a process for deriving useful energy from produced geothermal brine in which waste water can be efficiently reinjected into a subterranean geothermal reservoir in a manner which is not deleterious to the reinjection piping and well, nor to the geothermal brine within the reservoir.
These and other objects and advantages of the invention will be apparent from the following detailed description and drawing.